Object-Oriented Programming (OOP) Lecture No. 25

Overriding Member Functions of Base Class Derived class can override the member functions of its base class To override a function the derived class simply provides a function with the same signature as that of its base class

Overriding Parent ... Func1 Child ... Func1

Overriding class Parent { public: void Func1(); void Func1(int); }; class Child: public Parent { void Func1();

Overloading vs. Overriding Overloading is done within the scope of one class Overriding is done in scope of parent and child Overriding within the scope of single class is error due to duplicate declaration

Overriding class Parent { public: void Func1(); void Func1(); //Error };

Overriding Member Functions of Base Class Derive class can override member function of base class such that the working of function is totally changed

Example class Person{ public: void Walk(); }; class ParalyzedPerson: public Person{

Overriding Member Functions of Base Class Derive class can override member function of base class such that the working of function is similar to former implementation

Example class Person{ char *name; public: Person(char *=NULL); const char *GetName() const; void Print(){ cout << “Name: ” << name << endl; } };

Example class Student : public Person{ char * major; public: Student(char * aName, char* aMajor); void Print(){ cout <<“Name: ”<< GetName()<<endl << “Major:” << major<< endl; } ... };

Example int main(){ Student a(“Ahmad”, “Computer Science”); a.Print(); return 0; }

Output Output: Name: Ahmed Major: Computer Science

Overriding Member Functions of Base Class Derive class can override member function of base class such that the working of function is based on former implementation

Example class Student : public Person{ char * major; public: Student(char * aName, char* m); void Print(){ Print();//Print of Person cout<<“Major:” << major <<endl; } ... };

Example int main(){ Student a(“Ahmad”, “Computer Science”); a.Print(); return 0; }

Output There will be no output as the compiler will call the print of the child class from print of child class recursively There is no ending condition

Example class Student : public Person{ char * major; public: Student(char * aName, char* m); void Print(){ Person::Print(); cout<<“Major:” << major <<endl; } ... };

Example int main(){ Student a(“Ahmad”, “Computer Science”); a.Print(); return 0; }

Output Output: Name: Ahmed Major: Computer Science

Overriding Member Functions of Base Class The pointer must be used with care when working with overridden member functions

Example int main(){ Student a(“Ahmad”, “Computer Scuence”); Student *sPtr = &a; sPtr->Print(); Person *pPtr = sPtr; pPtr->Print(); return 0; }

Example Output: Name: Ahmed Major: Computer Science

Overriding Member Functions of Base Class The member function is called according to static type The static type of pPtr is Person The static type of sPtr is Student

Hierarchy of Inheritance We represent the classes involved in inheritance relation in tree like hierarchy

Example GrandParent Parent1 Parent2 Child2 Child1

Direct Base Class A direct base class is explicitly listed in a derived class's header with a colon (:) class Child1:public Parent1 ...

Indirect Base Class An indirect base class is not explicitly listed in a derived class's header with a colon (:) It is inherited from two or more levels up the hierarchy of inheritance class GrandParent{}; class Parent1: public GrandParent {}; class Child1:public Parent1{};